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THIS SNOWPACK SUMMARY EXPIRED ON March 15, 2017 @ 1:12 amSnowpack Summary published on March 14, 2017 @ 8:12 am

Issued by
Ben Bernall - Kootenai National Forest

Avalanche Character 1: Wet Slab

Wet Slab avalanches occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoid avalanche terrain when you see pinwheels, roller balls, loose wet avalanches, or during rain-on-snow events.

Yesterdays tour revealed a stable snowpack as a result of last weekends warming to cooling trend. I expect this to change today as the current mountain weather forecast is calling for more warm temperatures and rain up to 8,000'. These conditions will likely make any weak layers in the snowpack reactive and dangerous until things cool back down on Thursday.

Bottom Line: Spring is coming. Watch the temperatures and choose conservative terrain during these warm days and wait for the temperatures to drop back below the freezing mark before pushing into steep terrain.

Avalanche Character 2: Loose Wet

Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

The new snow that sits on various crust layers will cave into gravity as rain and temperatures rise. These slow moving wet slides often appear benign but can have hazardous consequences if they carry you into cliffs or trees.

Bottom Line: Again, be conservative in your terrain selection on warm days. Gullies, chutes and cliffs can become dangerous obstacles when loose, wet snow starts to run off the mountian.

Avalanche Character 3: Persistent Slab

Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

This problem is mainly a concern in the Purcell Range of the Yaak. I will continue talking about it due to the fact that it harbors some nasty weak layers that may persist for the season. These warm days will reduce the strength of the upper snowpack, rain will add weight and these deep, stubborn weaknesses will become an issue under the warm forecasted conditions.

Snowpack Discussion

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On March 13th we travelled into the high country of Chicago Peak in the Eastern Cabinets. The warm temperatures from the previous weekend were followed by cooler temps above 5,000' with light amounts of new snow. These conditions resulted in a quite stable snowpack at upper elevations where the cooler weather had locked the snowpack back together. The most notable weakness was a layer of graupel that was 8" below the surface. It failed with moderate force but is not showing a strong likelihood to propagate into a slabavalanche. We travelled as high as 7,000' where we noted some strong winds at 10-15 mph. There was little snow available for transport and we did not observe any signs of wind slab forming at upper elevations.

DISCUSSION: Moisture will continue to flow over the Northern
Rockies for the next several days. Models have stayed relatively
consistent on their depiction of the event, with totals from
tonight through Thursday still on the order of 2 to 3 inches of
liquid equivalent in the terrain. Snow levels will continue to
rise to 7000 to 8000 ft today, staying there through Wednesday. On
Thursday morning, snow levels drop to 3500 to 4500 behind a cold
front. By Thursday evening, the atmosphere dries out, with the
next round of precipitation coming in Saturday morning for the MT-
ID border.

This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.

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